Tilted, or blazed Bragg gratings formed in optical waveguides, and in particular—in optical fibers are known for their ability to out-couple light propagating in the waveguide into a leaky mode exiting the waveguide in one or more locations along the waveguide length, thus providing an optical tap or taps. In many optical systems, such a grating-based optical tap is useful in capturing and monitoring a signal passing through the optical waveguide.
U.S. Pat. No. 5,061,032, issued to G. Meltz et al., discloses a particular optical fiber tap arrangement that utilizes a blazed, chirped refractive index grating selected to redirect light guided in the fiber such that it comes to a focus at a point outside of the fiber. The patent also discloses that the angle of the external path of the redirected light to the fiber axis that results in the constructive interference is peculiar to the respective central wavelength (λ).
The wavelength-dependent tap taught by Meltz et al required a relatively large blaze angle, preferably at least 22 degrees, to achieve the desired redirection of the light guided in the fiber core to light in space outside of the fiber. This relatively large blaze angle was found to result in polarization sensitivity, when the fraction of light that is redirected by the grating depends on the polarization of the incident guided light. U.S. Pat. Nos. 5,832,156, 5,850,302 and 6,002,822 issued to Strasser et al disclose dispersive optical taps for wavelength monitoring, wherein the undesirable polarization sensitivity of the tapped fraction was reduced by using blazing angles that are smaller than 15 degrees for coupling the guided mode to one or more cladding modes. Appropriate coupling means, comprising e.g. a glass prism, are further provided for coupling the cladding modes to radiation modes and for dispersing the out-coupled light along the fiber axis in dependence on the wavelength; an array of photodetectors disposed outside of the waveguide along the fiber axis is provided for determining spectral content of the guided mode in dependence on the light signal location along the array.
One of the drawback of the dispersive tap taught by Meltz et al is that it requires special optical coupling or optical beam modifying elements, in addition to the blazed grating, to out-couple light from the optical fiber. A recent U.S. Pat. No. 6,885,792 issued to Eggleton et al, discloses a wavelength monitoring system comprising a series of discrete blazed gratings for first spatially separating individual wavelength bands within the optical fiber along its axis and for out-coupling these spatially-separated bands from the fiber guiding mode to radiation modes, and then detecting theses out-coupled bands in the near field using inexpensive detecting apparatus, without any, or at least most, optically modifying elements. However, this solution requires forming multiple gratings along the fiber length, which increases the tap size and grating writing complexity.
Several prior-art fiber-optic devices exploit the polarization sensitivity of the light out-coupled by a blazed, or tilted, Bragg grating having a large tilt angle. These devices employ tilted fiber Bragg gratings that out-couple light in different azimuthal directions about the fiber axis. For example, U.S. Pat. No. 6,211,957 to Edorgan discloses an in-line polarimeter comprising four consecutive blazed grating inscribed with four different orientations to the fiber axis (0.degree., 90.degree. and 45.degree., 135.degree.) and UV-induced waveplates, for determining four Stokes parameters. Since each grating in this device out-couples light in one azimuthal direction, a relatively complex gratings and photodiodes arrangement is required for the polarimeter to function. U.S. Pat. No. 6,591,024 to Westbrook discloses an in-line fiber device using blazed gratings that combines the wavelength sensitivity of the in-plane, or longitudinal out-coupling angle with azimuthal polarization sensitivity to provide a combined spectrometer/polarimeter fiber device. In all these devices, each of the Bragg gratings couples light predominantly in one azimuthal direction defined by the grating tilt orientation within the fiber, with the coupling efficiency dependent on the tilt orientation relative to a polarization plane of the incident light.
Recently, it was shown that a blazed Bragg fiber grating can be formed in an optical fiber so that it out-couples monochromatic light simultaneously in two different azimuthal directions, or two spatially separated intensity maxima, which were found to be sensitive to the light polarization (J. Peupelmann, E. Krause, A. Bandemer and C. Schäffer, “Fibre-polarimeter based on grating taps,” Electron. Lett. 38, 1248-1250, 2002). This effect was attributed by the authors to the lens effect of the fibre surface leading to differently orientated grating regions in the fibre core during the grating inscription process, resulting in the coupling out of different states of polarization (SOPs) in different directions, which can be viewed as a simultaneous formation of two blazed gratings in the fiber during the grating inscription. U.S. Pat. No. 6,816,260 issued to Peupelmann et al discloses an in-line polarimeter that exploits polarization sensitivity of these intensity maxima to reduce the number of blazed gratings required for performing polarization state measurements. However, the inventors did not provide any teaching of wavelength sensitivity of the azimuthal separation of the maxima.
An object of the present invention is to provide an in-line fiber-optic spectrometer that uses azimuthal distribution of out-coupled light for spectral measurements.
It is another object of this invention to provide a compact waveguide Bragg grating device for measuring a characteristic of light propagating therein using wavelength dispersion of the light coupled out of the waveguide in a transverse to the waveguide direction.
It is another object of this invention to provide an in-line tilted Bragg grating based fiber-optic spectrometer having a photodetector array positioned normally to the fiber axis.